In a typical in-line shrink wrap packaging machine, one or more unpackaged items are moved towards a shrink wrap cutting and sealing area by an in-feed conveyor. As the item or items are moved into the shrink wrap cutting and sealing area, the item or items are surrounded by center-folded shrink wrap film that has been partially unfolded so that the item or items can be conveyed into the partially unfolded center-folded film. When the item or items have been conveyed into the partially unfolded center-folded film, the item or items exit the in-feed conveyor and are transferred to an exit conveyor. It is typical for the exit conveyor to continue conveying the item or items until the item or items are moved to the shrink wrap cutting and sealing area beyond a designated cross-sealing location. At that point, it is typical to stop forward movement of the exit conveyor, and use a hot knife system to cut the layers of the shrink wrap film and contemporaneously weld or seal the layers of film together at the cut so that the shrink wrap film is wrapped around the item or items.
After the shrink wrap film has been cut and sealed the exit conveyor is again activated, and the item or items are moved into a shrink tunnel or oven. After each group of one or more items is heated in the shrink tunnel, the shrink wrap tightens around the item or items so that the item or items are securely wrapped.
The cutting and sealing is repeated on consecutive groups of one or more items as the groups are conveyed through the system. The cut and seal across the film in between consecutive groups of one or more items being separately wrapped is a cross seal. The cut and seal along the length of the film on the side of the group of one or more items opposite from the center fold is a side seal. The selvage film that is wasted beyond the side seal is disposed of by winding on selvage rollers.
An L-shaped jaw-type hot knife systems can be used to contemporaneously cross-seal and side seal when the center-folded film is stopped. In a jaw-type system, the L-shaped hot knife is normally coated with Teflon, so that melted shrink wrap film will not stick to the knife. When the group of one or more items to be wrapped has been moved in the cutting and sealing area beyond the designated cross-sealing location, forward movement is stopped and the L-shaped hot knife presses down on a corresponding L-shaped seal pad, and cuts and seals the shrink wrap film. The L-shaped seal pad typically has a flat top surface that is covered with Teflon coated tape.
The dimensions of L-shaped hot knives must be customized to the size of the items being wrapped. It is important that a Teflon coated hot knife be extremely straight and without nicks. If the knife is improperly adjusted, warped, or has nicks, the shrink wrap will not seal properly. For instance, a nick in the knife will created a hole in the seal. Likewise, a warped or improperly adjusted knife will create an incomplete seal. When long items are being wrapped, these problems are exacerbated. In addition, long items require an extremely long side-sealing areas and an extremely long side-sealing hot knife. Such machines typically must be custom made.
Efforts have been made with rotary-type, hot knife side sealing systems that can cut and side seal shrink wrap film without stopping the forward movement of the film. Such rotary-type, hot knife side sealing systems typically have a circular Teflon coated knife mounted for rotation. Such systems are not well suited for high speed applications because of insufficient contact time for proper welding with the circular rotating knife.
Hot drag wire side sealers (e.g. film is dragged between two hot wires) can also be used, but such systems require high maintenance and produce inconsistent seals.